The present invention relates to a process by which binuclear 4,4-di-phenylmethane dicarbamates serving as precursors of 4,4'-diphenylmethane diisocyanate (4,4'-MDI or commonly referred to as "pure MDI") can be produced with high selectivity and in high yield by reacting N-phenyl carbamates with a methylenating agent. The demand at the present time for 4,4'-diphenylmethane diisocyanate (pure MDI) has rapidly increased since it can be used as a starting material for various products including adhesives, paints, spundex fibers and urethane elastomers. Under these circumstances, a great benefit would result from providing a process in which 4,4'-diphenylmethane dicarbamates serving as starting materials for pure MDI can be produced in an industrially advantageous way.
A conventional known process for producing diphenylmethane dicarbamates comprises reacting N-phenyl carbamates with methylenating agents such as trioxane, formalin (aqueous solution of formaldehyde), dimethoxymethane, paraformaldehyde and diacetoxymethane in the presence of acid catalysts such as organic sulfonic acids, polysulfuric acid, mineral acids, Lewis acids on graphite, sulfonate based cation exchangers, heteropolyphosphoric acid, clay minerals, and metal phosphates.
Various versions of this process have been proposed and in one example, the reaction is carried out in the presence of organic solvents such as nitrobenzene, benzene and sulfolane using as acid catalysts, a variety of Bronsted and Lewis acids including trifluoromethanesulfonic acid, 96% sulfuric acid, fluorinated sulfonic or carboxylic acid resins, solid sulfuric acid, iron chloride on graphite, boron trifluoride and 40% ferric chloride (see Unexamined Published Japanese Patent Application Nos. 57550/1980, 115862/1980, 129260/1980, 160012/1980, 171952/1982, 171953/1982, 171954/1982 and 62151/1983). However, due to the high acidic strength of the catalysts used, the yield of the 4,4'-diphenylmethane dicarbamates produced is as low as 30-50% and tri- and more nuclear polymethylenepolyphenyl carbamates, namely, polynuclear compounds represented by the general formula: ##STR1## (R: alkyl, aralkyl or phenyl group) are formed as by-products in large quantities. Hence, this approach is not satisfactory for producing suitable starting materials for pure MDI.
With a view to solving this problem, it has been proposed to use aqueous acid solutions having concentrations of 10 wt% and higher (Unexamined Published Japanese Paten& Application Nos. 81850/1980 and 81851/1980) or trifluoromethanesulfonic acid in aqueous solvents (Unexamined Published Japanese Patent Application No. 79358/1980). In these methods, the strength of acid catalysts is sufficiently reduced by the presence of water to bring about a preferred result in that the formation of the tri- and more nuclear polymethylenepolyphenyl carbamates as by-products is suppressed. On the other hand, the reaction rate decreases to cause difficulty in completing the reaction and compounds having a methyleneamino bond (--CH.sub.2 --N--) that are precursors of 4,4'-diphenylmethane dicarbamates, namely, intermediates represented by the general formula: ##STR2## (R: alkyl, aralkyl or phenyl group) remain in large quantities in the reaction solution.
It is difficult to separate the compounds having a methyleneamino bond from 4,4'-diphenylmethane dicarbamates If the mixture of these compounds is decomposed thermally, diphenylmethane diisocyanate would be generated from 4,4'-diphenylmethane dicarbamates but no isocyanate would form from the compounds having a methyleneamino bond. These compounds do not merely remain unreacted and it has been pointed out that these compounds would cause various side reactions with isocyanate to substantially lower the yield of the desired isocyanate (Unexamined Published Japanese Patent application No. 106453/1984).
In order to insure that no compounds having a methyleneamino bond would remain unreacted in the reaction solution, it has been proposed that after condensing N-phenyl carbamates with a methylenating agent in the presence of an aqueous solution of an inorganic acid as a catalyst, the catalyst should be separated and the organic reaction mixture be subjected to further reaction in the presence of a carboxylic acid (pKa.ltoreq.4) catalyst so that the compounds having a methyleneamino bond are isomerized to 4,4'-diphenylmethane dicarbamates (Unexamined Published Japanese Patent Application No. 106453/1984).
This method not only achieves the intended object (i.e., producing a reaction solution that does not contain any intermediate compounds having a methyleneamino bond) but also suppresses the formation of the by-product, polymethylenepolyphenyl carbamate. Nevertheless, the reaction would unavoidably proceed in two stages of condensation and isomerization and this greatly adds to the complexity of the production process. Further, carboxylic acids of pKa.ltoreq.4 typified by strong acids such as trifluoroacetic acid are used as isomerization catalysts, so in order to protect against the corrosive action of such acids, the reactor must be made of an expensive material and this would increase the cost of producing the desired pure MID.